Glass substrate for information recording medium and magnetic information recording medium to which the glass substrate is applied

ABSTRACT

Disclosed are a glass substrate for an information recording medium, having excellent scratch resistance and a light weight and having high fracture toughness, the glass substrate having a fragility index value, measured in water, of 12 μm −1/2  or less or having a fragility index value, measured in an atmosphere having a dew point of −5° C. or lower, of 7 μm −1/2  or less, or the glass substrate comprising, by mol %, 40 to 75% of SiO 2 , 2 to 45% of B 2 O 3  and/or Al 2 O 3  and 0 to 40% of R′ 2 O in which R′ is at least one member selected from the group consisting of Li, Na and K), wherein the total content of SiO 2 , B 2 O 3 , Al 2 O 3  and R′ 2 O is at least 90 mol %, and a magnetic information recording medium comprising a magnetic recording layer formed on the glass substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a glass substrate for an informationrecording medium and a magnetic information recording medium to whichthe glass substrate is applied. More specifically, the present inventionrelates to a glass substrate for an information recording medium, whichglass substrate is excellent in scratch resistance and light in weightand has high fracture toughness, and a magnetic information recordingmedium to which the above glass substrate is applied, which medium istypified by a hard disk drive.

2. Prior Art of the Invention

Conventionally, aluminum, glass, ceramic, or the like is used as asubstrate material for a magnetic information recording medium. Atpresent, aluminum and glass are mainly practically used depending upon asize and use. Of these, a glass substrate has no or few surface defectsand is excellent in smoothness and surface hardness, so that the extentto which it is used is expanding year after year. As a glass for use asa substrate for a magnetic information recording medium, a chemicallystrengthened glass according to an ion-exchange method, a glass ceramic,etc., are known. As a chemically strengthened glass, for example,JP-A-1-239036 discloses a chemically strengthened glass substrate for amagnetic disk, which substrate is formed of a glass containing, byweight %, 50 to 65% of SiO₂, 0.5 to 14% of Al₂O₃, 10 to 32% of R₂O (inwhich R is an alkali metal ion), 1 to 15% of ZnO and 1.1 to 14% of B₂O₃and having a compressive stress layer formed in the surface thereof byan ion-exchange method. Further, as a crystallized glass, for example,U.S. Pat. No. 5,391,522 discloses a glass ceramic substrate for amagnetic disk, which glass ceramic substrate contains, by weight %, 65to 83% of SiO₂, 8 to 13% of Li₂O, 0 to 7% of K₂O, 0.5 to 5% of MgO, 0 to5% of ZnO, 0 to 5% of PbO, provided that MgO+ZnO+PbO=0.5 to 5%, 1 to 4%of P₂O₅, 0 to 7% of Al₂O₃ and 0 to 2% of As₂O₃+Sb₂O₃, the above glassceramic substrate containing fine Li₂O.2SiO₂ as a main crystal.

In recent years, however, an information recording device such as amagnetic disk drive typified by a hard disk drive is demanded to satisfya higher recording density and a higher speed of writing and readingdata, so that it is required to increase the rotation of a disk. Therotation of a disk is approximately 7,200 rpm at present, and it isexpected that the rotation will be 15,000 rpm or larger in the future.In particular, a hard disk drive for a server for processing a largevolume of data will be particularly increasingly required to satisfysuch a demand. With an increase in the rotation of a recording medium,however, the recording medium is caused to bend and a resonance grows,so that there is increased a risk that the surface of the recordingmedium collides with a magnetic head to cause a read error or to causeclashing of the magnetic head. In an existing recording medium,therefore, it is difficult to decrease a distance (flying height)between the magnetic head and the recording medium beyond a certainvalue, so that the above risk is constituting a factor that inhibits anincrease in the recording density of a magnetic recording device. Theabove bending-resonance problem of the recording medium can be overcomeby employing a substrate material having a high elastic coefficient.

However, a conventional aluminum substrate has an elastic coefficient ofapproximately 72 GPa, and a glass substrate has an elastic coefficientof approximately 80 to 100 GPa, so that these substrates cannot complywith an increase in the rotation. There is therefore a trend towardincreasing the thickness of a substrate to comply with an increase inthe rotation. An increase in the thickness of a substrate involves anincrease in weight, so that the power consumption for an increase in therotation increases. Substrate materials having a smaller weight than analuminum alloy having a large density (2.76 g/cm³) are commerciallydemanded. Further, since an aluminum substrate has a far lower surfacehardness than a glass substrate and is therefore liable to undergoplastic deformation, so that the surface of a recording medium may bedented due to a collision between a high-speed revolving substrate and amagnetic head. A glass substrate is excellent over an aluminum substratein all of elastic coefficient, surface hardness and surface smoothness.However, the glass substrate is more fragile than the aluminumsubstrate, and a slight scratch formed during its production steps leadsto breakage of the glass substrate. For example, when a glass is usedfor a magnetic disk-substrate, the formation of the magnetic diskrequires many working processes such as processing to form a circularform, making of a center hole and processing of surfaces of inner andouter circumferences. During the above processing steps, many scratchesthat can be start points of fractures occur in a glass edge portion,etc., and slight scratches that are formed not only during theproduction step but also during handlings such as mounting of the glassto a spindle, etc., may lead to the breakage of the substrate. The aboveproblem gains an importance with an increase in the rotation of amagnetic disk. For overcoming these problems, it comes to be required toprovide a substrate glass having a small density and having little or nosuceptibility to scratches or a substrate glass having high resistanceto progress of glass fracture, that is, high fracture toughness.

SUMMARY OF THE INVENTION

Under the circumstances, it is an object of the present invention toprovide a glass substrate for an information recording medium, which hasa small density and has excellent scratch resistance so that it islittle or not at all susceptible to scratches, and further, which hashigh resistance against progress of fracture, that is, high fracturetoughness, as well as a magnetic information recording medium to whichthe above glass substrate is applied.

For achieving the above object, the present inventors have made diligentstudies and as a result have found that a glass substrate having afragility index value, measured in water and/or a dry atmosphere, of acertain value or smaller or a glass substrate having a specific glasscomposition can suit the above object as a glass substrate for aninformation recording medium, and the present invention has beenaccordingly completed on the basis of the above findings.

That is, the present invention provides:

(1) a glass substrate for an information recording medium, having afragility index value, measured in water, of 12 μm^(−1/2) or less (to bereferred to as “glass substrate I for an information recording medium”hereinafter),

(2) a glass substrate for an information recording medium, having afragility index value., measured in an atmosphere having a dew point of−5° C. or lower, of 7 μm^(−1/2) or less (to be referred to as ∂glasssubstrate II for an information recording medium” hereinafter),

(3) a glass substrate for an information recording medium, having afragility index value, measured in water, of 12 μm^(−1/2) or less andhaving a fragility index value, measured in an atmosphere having a dewpoint of −5° C. or lower, of 7 μm^(−1/2) or less (to be referred to as“glass substrate III for an information recording medium” hereinafter),

(4) a glass substrate for an information recording medium as recited inthe above (1), (2) or (3), comprising, by mol %, greater than 65%, as atotal, of SiO₂ and at least one of B₂O₃ and Al₂O₃, 0 to 20% of RO inwhich R is at least one member selected from the group consisting of Ma,Ca, Zn, Sr and Ba, 0 to 28% of R′₂O in which R′ is at least one memberselected from the group consisting of Li, Na and K, 0 to 10% of TiO₂ and0 to 10% of ZrO₂, the total content of said components being at least 95mol %,

(5) a glass substrate for an information recording medium, comprising,by mol %, 40 to 75% of SiO₂, 2 to 45% of B₂O₃ and/or Al₂O₃ and 0 to 40%of R′₂O in which R′ is at least one member selected from the groupconsisting of Li, Na and K), wherein the total content of SiO₂, B₂O₃,Al₂O₃ and R′₂O is at least 90 mol % (to be referred to as “glasssubstrate IV for an information recording medium” hereinafter),

(6) a glass substrate for an information recording medium as recited inthe above (5), having a fragility index value, measured in water, of 12μm^(−1/2) or less.

(7) a glass substrate for an information recording medium as recited inthe above (5) or (6), having a fragility index value, measured in anatmosphere having a dew point of −5° C. or lower, of 7 μm^(−1/2) orless,

(8) a glass substrate for an information recording medium as recited inany one of the above (1) to (7), having a Young's modulus of at least 70GPa,

(9) a glass substrate for an information recording medium as recited inany one of the above (1) to (8), having a modulus of rigidity of atleast 20 GPa,

(10) a glass substrate for an information recording medium as recited inany one of the above (1) to (9), which is made of a glass having aregion wherein the glass has a viscosity of at least 1 Pa·s, in a rangeof temperatures equivalent to, and higher than, a liquidus temperatureof the glass,

(11) a glass substrate for an information recording medium as recited inany one of the above (1) to (10), which is made of a glass having athermal expansion coefficient of 60×10⁻⁷/° C. or greater at atemperature of from 100° C. to 300° C.,

(12) a glass substrate for an information recording medium as recited inany one of the above (1) to (11), which has no chemically strengthenedlayer,

(13) a glass substrate for an information recording medium as recited inany one of the above (1) to (11), which has a chemically strengthenedlayer, and

(14) a magnetic information recording medium comprising a magneticrecording layer formed on the glass substrate for an informationrecording medium recited in any one of the above (1) to (13).

PREFERRED EMBODIMENTS OF THE INVENTION

The glass substrate for an information recording medium, provided by thepresent invention, includes four embodiments, that is, glass substratesI to IV for an information recording medium.

The glass substrate I for an information recording medium, provided bythe present invention, is a glass substrate having a fragility indexvalue, measured in water, of 12 μm^(−1/2) or less. With a decrease inthe fragility index value in water, the glass substrate comes to be lessfragile. The fragility index value in water is preferably 10.5 μm^(−1/2)or less, more preferably 9 μm^(−1/2) or less, still more preferably 8μm^(−1/2) or less. When the fragility index value of the glass substratein water is of the above value, the glass substrate is free fromfracture during its polishing in a state where the glass substrate issoaked in a polishing liquid, or during its handling in a state where itis wet with a polishing liquid or a washing liquid.

The glass substrate II for an information recording medium, provided bythe present invention, is a glass substrate having a fragility indexvalue, measured in an atmosphere having a dew point of −5° C. or lower,of 7 μm^(−1/2) or less. The above fragility index value is preferably 6μm^(−1/2) or less, more preferably 5 μm^(−1/2) or less, still morepreferably 4 μm^(−1/2) or less. When the fragility index value in anatmosphere having a dew point of −5° C. or lower, that is, in a dryatmosphere, is of the above value, the glass substrate is free fromfracture during its handling in a dry atmosphere or during the use of aninformation recording medium to which the glass substrate is applied.

The glass substrate III for an information recording medium, provided bythe present invention, is a glass substrate having a fragility indexvalue, measured in water, of 12 μm^(−1/2) or less and having a fragilityindex value, measured in an atmosphere having a dew point of −5° C. orlower, of 7 μm^(−1/2) or less. The fragility index value thereof inwater is preferably 10.5 μm^(−1/2) or less, more preferably 9 μm^(−1/2)or less, still more preferably 8 μm^(−1/2) or less. Further, thefragility index value thereof in an atmosphere having a dew point of −5°C. or lower is preferably 6 μm^(−1/2) or less, more preferably 5μm^(−1/2) or less, still more preferably 4 μm^(−1/2) or less. The glasssubstrate III has properties that both the above glass substrates I andII have, and it is free from fracture during its use or its handling inany atmosphere.

With regard to the above fragility index value of a glass substrate, thepresent invention refers to a fragility index value B proposed by B. R.Lawn, et al (Journal of the American Chemical Society, Vol. 62, pages347 to 350 (1979)). The fragility index value B is defined on the basisof the following equation.B=Hv/Kc

wherein Hv is a Vickers hardness value and Kc is a fracture toughnessvalue.

The Vickers hardness value Hv and the fracture toughness value Kc of aglass can be measured by a method in which an acute diamond presser of aVickers hardness tester is pressed into the glass. That is, these valuescan be determined on the basis of the following equation using a pressedmark remaining in the glass surface when the Vickers presser is pressedinto the glass.${Hv} = {1.8544 \times \frac{P}{\left( {2a} \right)^{2}}}$

wherein P is a load applied for pressing the Vickers presser, and a is adiagonal length of a Vickers pressed mark. The fracture toughness Kc isdetermined according to the following equation using the size of apressed mark remaining in a glass surface after the Vickers presser ispressed into the glass and the length of a crack that occurs in a cornerof the pressed mark.${Kc} = {0.026\frac{E^{\frac{1}{2}}P^{\frac{1}{2}}a}{C^{\frac{3}{2}}}}$

wherein E is a Young's modulus of the glass, and C is a length of acrack that occurs in a corner of the pressed mark. The necessarycondition of determining a correct Kc is that the ratio of C/a comes tobe 2.5 or greater.

The fragility index value B of the glass can be calculated on the basisof B=Hv/Kc using the above-determined Hv and Kc values. However, thepresent invention employs values calculated according to the methoddescribed in JP-A-10-158028, for comparisons with Comparative Examplesto be described later.

That is, the present invention employs values calculated according tothe following equation.$B = {2.39 \times \left\lbrack \frac{C}{\alpha} \right\rbrack^{\frac{3}{2}}P^{\frac{1}{4}}}$

The fragility value calculated according to the equation shown inJP-A-10-158028 and the fragility value calculated according to the Hv/Kcequations differ from each other mostly by 5% or less, so that any glasscan be correctly evaluated according to the equation shown inJP-A-10-158025. The equation shown in JP-A-10-158028 is actuallyintroduced from the Hv/Kc equations of Lawn, et al., and thesecalculations are basically the same in concept.

The fragility index value in water refers to a value obtained bydropping pure water on a sample surface, pressing a Vickers presser intothe sample surface through a water drop 30 seconds thereafter, to form apressed mark and a crack, soaking the sample in pure water immediatelythereafter to retain the sample in the pure water for 24 hours, thentaking the sample out, wiping water off the sample, and measuring thesample for sizes of the pressed mark and the crack immediatelythereafter. The fragility index value in an atmosphere having a dewpoint of −5° C. or lower refers to a value obtained by measuring a dewpoint around a sample in a dry nitrogen atmosphere to confirm that thedew point is −5° C. or lower, pressing a Vickers presser whileconfirming the dew point of −5° C. or lower, to form a pressed mark anda crack and measuring the pressed mark and the crack for sizes.

The glass substrates I, II and III for an information recording medium,provided by the present invention, have the above fragility index value,and they are not easily scratched or damaged and can be chemicallystrengthened equally to a conventional glass, so that the fracturethereof during their production steps and during use thereof can bedecreased to a great extent.

The above glass substrates I to III can have a glass composition thatcomprises, by mol %, greater than 65%, as a total, of SiO₂ and at leastone of B₂O₃ and Al₂O₃, 0 to 20% of RO in which R is at least one memberselected from the group consisting of Ma, Ca, Zn, Sr and Ba, 0 to 28% ofR′₂O in which R′ is at least one member selected from the groupconsisting of Li, Na and K, 0 to 10% of TiO₂ and 0 to 10% of ZrO₂, thetotal content of said components being at least 95 mol %.

In the above glass composition, the total content of SiO₂ and at leastone of B₂O₃ and Al₂O₃ is preferably greater than 65 mol % but not morethan 90 mol %, more preferably in the range of from 70 to 90 mol %,still more preferably 70 to 85 mol %. The content of SiO₂ is preferably40 to 75 mol %, more preferably 50 to 70 mol %, and the content of Al₂O₃is preferably 0 to 25 mol %, more preferably 1 to 20 mol %, still morepreferably 2 to 15 mol %. The content of B₂O₃ is preferably 0 to 25 mol%, more preferably 1 to 25 mol %, still more preferably 2 to 20 mol %.The content of the above RO is preferably 15 mol % or less, morepreferably 12 mol % or less. The content of MgO is preferably 15 mol %or less, more preferably 12 mol % or less, and the content of CaO ispreferably 10 mol % or less, more preferably 8 mol % or less. Thecontent of ZnO is preferably 10 mol % or less, more preferably 8 mol %or less, and the content of SrO is preferably 10 mol % or less, morepreferably 8 mol % or less. The content of BaO is preferably 10 mol % orless, more preferably 5 mol % or less. The RO is preferably MgO.

The content of R₂′O is preferably 25 mol % or less, more preferably 10to 25 mol % or less. The content of Li₂O is preferably 20 mol % or less,more preferably 18 mol % or less, still more preferably 5 to 15 mol % orless, and the content of Na₂O is preferably 20 mol % or less, morepreferably 15 mol % or less, still more preferably 1 to 10 mol % orless. The content of K₂O is preferably 15 mol % or less, more preferably10 mol % or less, still more preferably 0 to 8 mol %. The total contentof SiO₂, B₂O₃, Al₂O₃, RO and R′₂O is preferably at least 85 mol %, morepreferably at least 90 mol %, still more preferably at least 95 mol %.

Further, the content of TiO₂ is preferably 0 to 7 mol %, more preferably0 to 5 mol %, and the content of ZrO₂ is preferably 0 to 8 mol %, morepreferably 0 to 6 mol %. The total content of SiO₂, B₂O₃, Al₂O₃, RO,R′₂O, TiO₂ and ZrO₂ is preferably at least 95 mol %, more preferably atleast 98 mol %. The molar ratio of B₂O₃/Al₂O₃ is preferably in the rangeof from 0.5 to 1.5, more preferably in the range of from 0.8 to 1.2.

As an example of combination of the above components, for example, theglass has a composition containing, by mol %, 40 to 75% of SiO₂, 1 to25% of B₂O₃, 1 to 20% of Al₂O₃ (provided that the total content of SiO₂,B₂O₃ and Al₂O₃ is over 65%), 0 to 15% of MgO, 0 to 10% of ZnO, 0 to 10%of CaO, 0 to 10% of SrO, 0 to 10% of BaO (provided that the totalcontent of MgO, CaO, ZnO, SrO and BaO is less than 20%), 0 to 20% ofLi₂O, 0 to 20% of Na₂O, 0 to 15% of K₂O (provided that the total contentof Li₂O, Na₂O and K₂O is less than 28%), 0 to 10% of TiO₂ and 0 to 10%of ZrO₂, provided that the total content of said components of thecomposition is at least 95 mol %.

SiO₂ is a component for forming a network structure of a glass. When thecontent thereof is less than 40 mol %, the durability of the glass ispoor, and the glass is liable to devitrify. When the content thereofexceeds 75 mol %, the high-temperature viscosity increases, so that theglass is not easily melted. Therefore, the content of SiO₂ is preferablyin the range of from 40 to 75 mol %, particularly preferably in therange of from 50 to 70 mol %.

B₂O₃ is an important component in the present invention. When B₂O₃ isintroduced in place of SiO₂, the fragility of the glass decreases to agreat extent, the specific gravity of the glass is also decreased, andthe high-temperature viscosity is decreased, so that the glass isremarkably improved in meltability. When the content of introduced B₂O₃exceeds 25 mol %, the durability of the glass is degraded, and the glassis liable to cause a phase separation, so that no good glass can beproduced in some cases. When the content of the introduced B₂O₃ is lessthan 1 mol %, the glass deteriorates in fragility, and thehigh-temperature viscosity increases, so that mass-production at a lowcost may not be carried out any longer. Therefore, the content of B₂O₃is preferably in the range of from 1 to 25 mol %, particularlypreferably 2 to 20 mol %.

Al₂O₃ is important not only as a component for imparting the glass withheat resistance and durability and improving the glass in fragility, butalso as a component for improving the glass in the stability of glassstructure and the rigidity of the glass together with SiO₂. When thecontent thereof is less than 1 mol %, there is produced no or littleeffect on suppressing alkali elution from the glass, and it is difficultto produce a glass having excellent durability. When the above contentis over 20 mol %, the high-temperature meltability of the glass isdeteriorated. Therefore, the content of Al₂O₃ is preferably in the rangeof from 1 to 20 mol %, more preferably in the range of from 2 to 15 mol%.

MgO, CaO, ZnO, SrO and BaO are introduced for decreasing the viscosityof the glass during its melting to improve the glass in meltability andmass-productivity. When the total content of these is over 20 mol %, thefragility increases, so that the glass is liable to be scratched ordamaged, and both the specific gravity and the devitrificationtemperature of the glass tend to increase. With regard to the content ofeach of MgO, CaO, ZnO, SrO and BaO, in view of both the meltability andthe fragility of the glass, the content of MgO is 0 to 15 mol %,preferably 0 to 12 mol %, the content of ZnO is 0 to 10 mol %,preferably 0 to 8 mol %, the content of CaO is 0 to 10 mol %, preferably0 to 8 mol %, the content of SrO is 0 to 10 mol %, preferably 0 to 8 mol%, the content of BaO is 0 to 10 mol %, preferably 0 to 5 mol %, and thetotal content of these components is preferably less than 20 mol %, morepreferably 15 mol % or less.

Li₂O, Na₂O and K₂O are very useful components for decreasing theviscosity of the glass during its melting to promote the melting, andfor decreasing the fragility of the glass. When the content of thesecomponents introduced is over 28 mol %, not only the glass isdeteriorated in chemical durability, but also an alkali is precipitatedon a glass surface to a greater extent, so that the alkali may corrode amagnetic film. With regard to the content of each of Li₂O, Na₂O and K₂O,therefore, the content of Li₂O is 0 to 20 mol %, preferably 0 to 18 mol%, the content of Na₂O is 0 to 20 mol %, preferably 0 to 15 mol %, thecontent of K₂O is 0 to 15 mol %, preferably 0 to 10 mol %, and the totalcontent of these components is controlled to less than 28 mol %,preferably to 25 mol % or less.

The total content of SiO₂, B₂O₃ and Al₂O₃ is preferably in the range offrom 70 to 90 mol %, more preferably 80 to 90 mol %, and the totalcontent of RO and R′₂O is preferably 5 to 35 mol %, more preferably 10to 30 mol %, still more preferably 10 to 25 mol %, particularlypreferably 10 to 22 mol %.

ZrO₂ and TiO₂ are introduced for improving the glass in chemicaldurability and rigidity. When a small amount of ZrO₂ and TiO₂ are addedto a glass, the glass is improved in all of durability, elasticity andfragility. However, the specific gravity of the glass sharply increases,and when they are introduced in a large amount, the glass strongly tendsto devitrify. The content of each of ZrO₂ is limited to 0 to 10 mol %,preferably to 0 to 7 mol %, and the content of TiO₂ is limited to 0 to10 mol %, preferably 0 to 8 mol %. Further, the total content of theabove components is at least 95 mol %.

In addition to the above components, the above glass may contain As₂O₃,Sb₂O₃, F, Cl and S0₃ in a total amount of 2 mol % or less for improvingthe glass in meltability, clarification and moldability. Further, theabove glass may contain other oxides, i.e., rare earth metal oxides suchas Y₂O₃ and La₂O₃ in an amount of 5 mol % or less, for improving theglass in durability and elastic modulus.

Further, the glass can also have a composition containing, by mol %, 55to 75% of SiO₂, 0 to 20% of B₂O₃, 1 to 20% of Al₂O₃ (provided that thetotal content of SiO₂, B₂O₃ and Al₂O₃ is at least 65%), 0 to 15% of MgO,0 to 10% of ZnO, 0 to 10% of CaO, 0 to 10% of SrO, 0 to 10% of BaO(provided that the total content of MgO, CaO, ZnO, SrO and BaO (contentof RO) is 20% or less) , 0 to 20% of Li₂O, 0 to 20% of Na₂O, 0 to 6% ofK₂O (provided that the total content of Li₂O, Na₂O and K₂O (content ofR′₂O) is 28% or less), 0to 10% of TiO₂ and 0 to 10% of ZrO₂, providedthat the total content of said components of the composition is at least95 mol %.

The glass substrate IV for an information recording medium, provided bythe present invention, is formed of a glass containing, by mol %, 40 to75% of SiO₂, 2 to 45% of B₂O₃ and/or Al₂O₃ and 0 to 40% of R′₂O in whichR′ is at least one member selected from the group consisting of Li, Naand K, and having an SiO₂+B₂O₃+R′₂O total content of at least 90 mol %.

In the above glass composition, the content of Sio₂ is preferably 50 to70 mol %, the content of B₂O₃ is preferably 0 to 25 mol %, morepreferably 1 to 25 mol %, still more preferably 2 to 20 mol %, and thecontent of Al₂O₃ is preferably 0 to 25 mol %, more preferably 1 to 20mol %, still more preferably 2 to 15 mol %. The total content of SiO₂,B₂O₃ and Al₂O₃ is preferably 65 to 90 mol %, more preferably 70 to 90mol %, still more preferably 70 to 85 mol %. The content of R′₂O ispreferably 0 to 28 mol % (provided that 0 is excluded when the contentof RO is 0 mol %), more preferably 25 mol % or less, still morepreferably 10 to 25 mol %. The content of Li₂O is preferably 20 mol % orless, more preferably 18 mol % or less, still more preferably 5 to 15mol %. The content of Na₂O is preferably 20 mole or less, morepreferably 15 mol % or less, still more preferably 1 to 10 mol %. Thecontent of K₂O is preferably 15 mol % or less, more preferably 10 mol %or less, still more preferably 0 to 8 mol %. Further, the content of ROis preferably 15 mol % or less, more preferably 12 mol % or less. Thecontent of MgO is preferably 15 mol % or less, more preferably 12 mol %or less, and the content of CaO is preferably 10 mol % or less, morepreferably 8 mol % or less. The content of ZnO is preferably 10 mol % orless, more preferably 8 mol % or less, and the content of SrO ispreferably 10 mol % or less, more preferably 8 mol % or less. Thecontent of BaO is preferably 10 mol % or less, more preferably 5 mol %or less. RO is particularly preferably MgO.

The total content of RO and R′₂O is preferably 5 to 35 mol %, morepreferably 10 to 30 mol %, still more preferably 10 to 25 mol %,particularly preferably 0 to 22 mol %. Further, 0 to 10 mol % of TiO₂may be contained, and the content of TiO₂ is preferably 0 to 7 mol %.Further, 0 to 10 mol % of ZrO₂ may be contained, and the content of ZrO₂is preferably 0 to 7 mol %.

The fragility index value of the glass substrate IV in water can beadjusted to 12 μm^(−1/2) or less, preferably to 10.5 μm-^(−1/2) or less,more preferably to 9 μm^(1/2) or less, still more preferably to 8μm^(−1/2) or less. Further, the fragility index value thereof in anatmosphere having a dew point of −5° C. or lower can be adjusted to 7μm^(−1/2) or less, preferably to 6 μm^(−1/2) or less, more preferably to5 μm^(−1/2) or less, still more preferably to 4 μm^(−1/2) or less.

In the glass substrates I to IV for an information recording medium,provided by the present invention, the Young's modulus of each can beadjusted to at least 70 GPa, preferably to 75 GPa, more preferably to 85GPa.

For preventing the deformation to be caused on the glass substrate dueto resonance when an information recording medium having the glasssubstrate having a small thickness is rotated, preferably, the Young'smodulus of the glass substrate is increased. For example, when amagnetic disk having a glass substrate having a diameter of 3.5 inchesand a thickness of 0.635 mm and made of a glass having a Young's modulusof 70 GPa or higher is rotated at 10,000 rpm, a flying height ofapproximately 1 μm or less can be stably secured between the magnetichead and a recording and reproducing head.

When a glass substrate having a low fragility index value in water has aYoung's modulus of at least 70 GPa, the glass substrate is remarkablyfree from the occurrence of a crack that may be caused when it is cut orpolished in a state where it is soaked in a polishing solution or whenit is handled in a state where it is wet with a polishing solution or awashing liquid.

Further, when a glass substrate having a low fragility index value inwater and/or in a dry atmosphere has a Young's modulus of at least 70GPa, the glass substrate does not easily suffer bending caused by arotation or a load, so that the glass substrate is further free from theoccurrence of a fracture that may be caused during the polishing of theglass substrate or the use of an information recording medium,particularly, when the information recording medium is rotated at a highspeed.

Further, in the glass substrates I to IV for an information recordingmedium, provided by the present invention, the rigidity can be adjustedto at least 20 GPa, preferably to at least 25 GPa, more preferably to atleast 30 GPa.

For preventing the deformation to be caused on the glass substrate dueto resonance when an information recording medium having the glasssubstrate having a small thickness is rotated, preferably, the rigidityof the glass substrate is increased. For example, when a magnetic diskhaving a glass substrate having a diameter of 3.5 inches and a thicknessof 0.635 mm and made of a glass having a rigidity of 20 GPa or higher isrotated at 10,000 rpm, a flying height of approximately 1 μm or less canbe stably secured between the magnetic head and a recording andreproducing head.

Further, When a glass substrate having a low fragility index value inwater has a rigidity of at least 20 GPa, the glass substrate isremarkably free from the occurrence of a crack that may be caused whenit is cut or polished in a state where it is soaked in a polishingsolution or when it is handled in a state where it is wet with apolishing solution or a washing liquid.

Further, when a glass substrate having a low fragility index value inwater and/or in a dry atmosphere has a rigidity of at least 20 GPa, theglass substrate-does not-easily suffer bending caused by a rotation or aload, so that the glass substrate is further free from the occurrence ofa fracture that may be caused during the polishing of the glasssubstrate or the use of an information recording medium, particularly,when the information recording medium is rotated at a high speed.

In the glass substrates I to IV for an information recording medium,provided by the present invention, the specific modulus (value obtainedby dividing a Young's modulus by a density) thereof can be adjusted toat least 27×106 N·m/kg.

When the above specific modulus is 27×10⁶ N·m/kg or more, the bendingduring the high-speed rotation of an information recording medium can bedecreased to 2 μm or less, and as a result, a flying height of 1 μm orless can be stably secured. Further, the bending during the high-speedrotation is decreased, so that a glass substrate having a smallfragility index value is more free from the occurrence of a fracture.The above specific modulus is more preferably 30×10⁶ N·m/kg or more.

In the glass substrates I to IV for an information recording medium,provided by the present invention, the density thereof can be adjustedto 2.65 g/cm³ or less, preferably to 2.50 g/cm³ or less.

Further, the glass substrates I to IV for an information recordingmedium, provided by the present invention, have a fracture toughnessvalue of at least 0.75 MPa/m^(1/2), preferably at least 0.80MPa/m^(1/2), more preferably at least 0.83 MPa/m^(1/2). When thefracture toughness is at least 0.75 MPa/m^(1/2), the glass substrate ismore free from the occurrence of a fracture during the processing of theglass substrate or during the use of an information recording medium.

The glass substrates I to IV for an information recording medium,provided by the present invention, are preferably made of a glass havinga region wherein the glass has a viscosity of at least 1 Pa·s, in arange of temperatures equivalent to, and higher than, the liquidustemperature of the glass.

For obtaining a glass substrate for an information recording medium, itis required to prevent a glass from undergoing substantialdevitrification during its production process. For this purpose, it isrequired at least to carry out the melting of raw materials and thefeeding of a glass melt to a mold at a liquidus temperature or higher.

In the glass substrates I to IV for an information recording medium,provided by the present invention, therefore, the liquidus temperaturethereof is preferably adjusted to 1,350° C. or lower, more preferably to1,250° C. or lower, particularly preferably to 1,150° C. or lower.

When a molten glass that is being fed to a mold has a viscosity of lessthan 1 Pa·s, not only it is difficult to control the flow rate of themolten glass, but also it is difficult to produce a thin and flat glasssubstrate for an information recording medium by press-molding.

More preferably, the glass substrates I to IV for an informationrecording medium, provided by the present invention, are made of a glasshaving a region wherein the glass has a viscosity of at least 3 Pa·s, ina range of temperatures equivalent to, and higher than, the liquidustemperature of the glass.

In the glass substrates I to IV for an information recording medium,provided by the present invention, the glass transition point of a rawmaterial glass can be adjusted to 470 to 640° C. When the glasstransition point is too high, a temperature range that permitspress-molding is narrowed, and it is difficult to press-mold a lasssubstrate having a small thickness. When the glass transition point istoo low, the temperature range for heat treatment for improving magneticproperties, etc., is narrowed, which heat treatment is carried out whenor after a magnetic film such as a recording layer is formed on theglass substrate. The glass transition point is preferably in the rangeof from 470 to 620° C.

The glass substrates I to IV for an information recording medium,provided by the present invention, are preferably made of a glass havinga thermal expansion coefficient of at least 60×10⁻⁷/° C. at atemperature of 100 to 300° C.

When information is recorded in an information recording medium such asa magnetic disk, an optical disk or a magneto-optical disk, or wheninformation recorded in the information recording medium is reproduced,the information recording medium rotates in a state where the it isclamp-fixed to the spindle of a drive motor provided in an informationprocessing device. When the thermal expansion coefficient of theinformation recording medium and the counterpart of the clamp aregreatly different, the following problem is caused.

That is, when the information recording medium is rotated, theinformation recording medium, the spindle, the clamp, etc., are sharplytemperature-increased up to approximately 90° C. due to the heatgeneration of the drive motor. When the thermal expansion coefficient ofthe information recording medium and the thermal expansion coefficientof the clamp differ greatly, the information recording medium and theclamp come loose due to the above increase in temperature, or theinformation recording medium is caused to suffer distortion or bending.As a result, a portion (track) where data is (to be) recorded in theinformation recording medium is displaced, and an error is liable tooccur in recording or reproducing information. The above problem comesto be conspicuous particularly in a large substrate such as a 3.5-inchsubstrate.

Therefore, the glass substrates I to IV for an information recordingmedium, provided by the present invention, preferably, the thermalexpansion coefficient thereof is as close to the thermal expansioncoefficient of the above clamp as possible. Since the above clamp isgenerally made of a stainless alloy, the thermal expansion coefficientin the range of 100 to 300° C. of each of the glass substrates I to IVprovided by the present invention is preferably at least 60×10⁻⁷/° C.,more preferably at least 70×10⁻⁷/° C., still more preferably 70×10⁻⁷/°C. to 120×10⁻⁷/° C., particularly preferably 80×10⁻⁷/° C. to 100×10⁻⁷/°C.

The glass substrates I to IV for an information recording medium,provided by the present invention, may have no chemically strengthenedlayer or may have a chemically strengthened layer formed by a knownchemical strengthening treatment as required. For carrying out thechemical strengthening treatment, it is preferred to select acomposition suitable for the chemical strengthening treatment from theafore-mentioned composition range.

The chemically strengthening treatment can be carried out by anion-exchange method. The ion-exchange method is carried out using amolten salt containing Na ion and K ion, and a chemically strengthenedglass is obtained. The treatment molten salt containing Na ion and K ionis preferably selected from sodium nitrate, potassium nitrate or amolten salt of a mixture of these, while it shall not be limitedthereto. A sulfate, a bisulfate, a carbonate or a halide may be used.The glass for use in the present invention has low fragility and highfracture toughness as described already, and comes to have high bendingstrength owing to then ion-exchange, so that the thus-obtainedchemically strengthened glass has excellent durability against fracture.

The method for producing the glass substrate for an informationrecording medium, provided by the present invention, is not speciallylimited, and various methods can be employed. For example, a glasssubstrate for an information recording medium, having a desired size anda desired form, can be produced by melting a predetermined amount ratioof glass raw materials in air or an inert gas atmosphere by ahigh-temperature melting method, homogenizing the glass by bubbling orstirring, forming a sheet glass by a known pressing method, down drawingmethod or floating method, and carrying out processing to form acircular form, making of a center hole, processing of surfaces of innerand outer circumferences, cutting and polishing. The polishing iscarried out by lapping with a polishing material or diamond pellets andpolishing with a polishing material such as cerium oxide, whereby asurface accuracy in a range of from 0.1 to 0.6 nm can be attained.

The magnetic information recording medium of the present inventioncomprises any one of the above glass substrates I to IV and a magneticrecording layer formed thereon. As an example of the constitution of themagnetic information recording medium, there is a constitution in whichan undercoat layer, a magnetic recording layer, a protective layer and alubricant layer are consecutively formed on the above glass substrate.

The above magnetic recording layer can be selected, for example, from aCo—Cr layer, a Co—Cr—Pt layer, a Co—Ni—Cr layer, a Co—Ni—Pt layer, aCo—Ni—Cr—Pt layer or a Co—Cr—Ta layer. The undercoat layer can beselected, for example, from an Ni layer, an Ni—P layer or a Cr layer. Asa protective layer, for example, a carbon film can be used, and for thelubricant layer, for example, a lubricant material such asperfluoropolyether can be used.

EXAMPLES

The present invention will be further specifically explained withreference to Examples, while the present invention shall not be limitedby these Examples.

Glasses obtained in Examples were measured for physical properties bythe following methods.

(1) Young's Modulus

A sample having a size of 20×20×100 mm was prepared, and the sample wasmeasured for a velocity of longitudinal wave (V1) and a velocity oftransverse wave when (Vs) when 5 MHz ultrasonic wave propagated throughthe above sample, with a sing around method acoustic velocity measuringapparatus (UVM-2, supplied by Cho-Onnpa Kogyosha), and a Young's moduluswas determined on the basis of the following equation.Young's modulus=(4G ²−3G·V1²·ρ)/(G−V1²·ρ)G=Vs ²·ρρ=sample density (g/cm³)

(2) Rigidity

When the measurement was made for a Young's modulus in the above (1), arigidity was obtained as G.

(3) Liquidus Temperature

A sample was placed in a container made of platinum, the container wasleft in a gradient-temperature furnace for 30 minutes, and the surfaceand the inside of the sample were observed for a crystal through anoptical microscope. And, a lowest temperature at which no crystalprecipitated was taken as a liquidus temperature.

(4) Glass Transition Point (Tg) and Sag temperature (Td)

A sample having a size of 5 mmφ×20 mm was measured at a temperatureelevation rate of +4° C./minute with a thermalmechanical analyzer (TMA8140) supplied by Rigakusha. SiO₂ was used as a reference sample.

(5) Thermal Expansion Coefficient

An average thermal expansion coefficient at 100 to 300° C. was employed,and the measurement thereof was conducted together with the measurementof glass transition point.

(6) Fragility index Value

With a microhardness tester (MVK-E) supplied by K. K. Akashi Seisakusho,a Vickers presser was pressed into a plate-shaped sample having athickness of 2 mm under a pressing load shown in Tables 1 to 15, tointroduce a pressed mark and a crack into the sample.

For accurate measurements for a fragility index value, a Vickershardness, a fracture toughness, etc., preferably, the pressing load isadjusted to such a value as to attain a probability of at least 60, morepreferably, to such a value as to attain a probability of at least 70,still more preferably, to such a value as to attain a probability of atleast 80. A Vickers pressed mark was measured for a diagonal length a,and a crack that occurred in a corner of the Vickers pressed mark causedin the sample surface when the Vickers presser was pressed was measuredfor a length C. A Vickers hardness Hv, a fracture toughness Kc and afragility index value B were determined using the above measurementvalues on the basis of the equations (1) to (3).

For determining a fragility index value B, a Vickers hardness Hv, afracture toughness Kc, etc., in water, a drop of pure water was droppedon the sample surface, and 30 seconds thereafter, the Vickers presserwas pressed into the sample to introduce a pressed mark and a crack.

Further, for determining a fragility index value B, a Vickers hardnessHv, a fracture toughness Kc, etc., in an atmosphere having a dew pointof −5° C. or lower, the Vickers presser was pressed into the sample tointroduce a pressed mark and a crack in a dry nitrogen atmosphere whilea dew point around the sample was −5° C. or lower.

Probability in Tables 1 to 14 refers to the probability of occurrences,per corner, of cracks that occur in four corners of a pressed mark.

Examples 1-81

SiO₂, Al₂O₃, Al(OH)₃, B₂O₃, HBO₃, MgO, Mg(OH)₂, MgCO₃, CaCO₃, SrCO₃,BaCO₃, ZnO, Li₂CO₃, K₂CO₃, TiO₂, ZrO₂, etc., were used as startingmaterials, and 300 to 1,500 g of each of the starting materials wasweighed for obtaining glass compositions shown in Tables 1 to 14 andfully mixed to prepare a formulated batch. The formulated batch wasplaced in a platinum crucible and melted at 1,400 to 1,600° C. in airfor approximately 3 to 8 hours to prepare a molten glass. The moltenglass was cast into a carbon mold having a size of 40×40×20 mm, andgradually cooled to a glass transition temperature, and the glass wasplaced in an annealing furnace immediately thereafter, held for 1 hourand then cooled to room temperature while being held in the furnace. Inthe thus-obtained glasses, no crystal that was observable through amicroscope was precipitated.

The thus-obtained glasses were processed to prepare samples forevaluations of various properties, and the samples were evaluated forphysical properties. Tables 1 to 14 show the results. TABLE 1 Examples 12 3 4 5 6 Composition SiO₂ 0.0 10.0 20.0 30.0 40.0 45.0 (mol %) B₂O₃60.0 50.0 40.0 30.0 20.0 10.0 Al₂O₃ 12.0 12.0 12.0 12.0 12.0 15.0 MgO8.0 8.0 8.0 8.0 8.0 10.0 CaO 15.0 20.0 20.0 15.0 10.0 10.0 ZnO — — — — —— RO 23.0 28.0 28.0 23.0 18.0 20.0 Li₂O — — — — — 5.0 Na₂O — — — 0.0 5.05.0 K₂O 5.0 0.0 0.0 5.0 5.0 0.0 R′₂O 5.0 0.0 0.0 5.0 10.0 10.0 TiO₂ — —— — — — ZrO₂ — — — — — — SiO₂ + B₂O₃ + Al₂O₃ 72.0 72.0 72.0 72.0 72.070.0 B₂O₃/Al₂O₃ 5.0 4.2 3.3 2.5 1.7 0.7 RO + R′₂O 28.0 28.0 28.0 28.028.0 30.0 SiO₂ + B₂O₃ + Al₂O₃ + RO + R′₂O 100.0 100.0 100.0 100.0 100.0100.0 SiO₂ + B₂O₃ + Al₂O₃ + R′₂O 77.0 72.0 72.0 77.0 82.0 80.0 SiO₂ +B₂O₃ + Al₂O₃ + RO + 100.0 100.0 100.0 100.0 100.0 100.0 R′₂O + TiO₂ +ZrO₂ Total 100.0 100.0 100.0 100.0 100.0 100.0 Glass transition pointTg[° C.] 552 612 622 590 565 554 Sag temperature Td [° C.] 596 660 664643 620 609 Thermal expansion coefficient α × 10⁻⁷[/° C.] 67 56 59 66 7967 Density [g/cm³] 2.373 2.490 2.522 2.472 2.478 2.547 Young's modulusE[GPa] 65.28 80 81.4 75.52 72.71 87.97 Rigidity G [GPa] 25.56 — — 30.2129.30 35.19 In water Pressing load [gF] 1000 1000 1000 1000 1000 1000Fragility index value B [μm^(−1/2)] 11.0 9.4 10.6 7.7 8.2 9.8 VickersHardness Hv [GPa] 4.6 5.3 5.8 5.3 5.2 6.1 Fracture toughness[MPa/m^(1/2)] 0.46 0.61 0.56 0.73 0.66 0.64 Probability 100 100 100 100100 100 In dry Pressing load [gF] 1000 1000 1000 1000 1000 1000atmosphere Fragility index value B [μm^(−1/2)] 6.0 5.0 6.1 6.0 5.6 5.9N₂ Vickers hardness Hv [GPa] — — — 5.5 5.1 6.1 Fracture roughness Kc[MPa/m^(1/2)] — — — 0.93 0.98 1.06 Probability — — — 100 40 100

TABLE 2 Examples 7 8 9 10 11 12 Composition SiO₂ 45.0 50.0 50.0 50.050.0 52.4 (mol %) B₂O₃ 20.0 10.0 10.0 10.0 20.0 23.8 Al₂O₃ 10.0 10.010.0 12.0 12.0 9.5 MgO 10.0 10.0 10.0 8.0 8.0 0.0 CaO 5.0 10.0 10.0 20.00.0 0.0 ZnO — — — — — — RO 15.0 20.0 20.0 28.0 8.0 0.0 Li₂O 0.0 0.0 5.0— 0.0 0.0 Na₂O 5.0 5.0 5.0 — 5.0 4.8 K₂O 5.0 5.0 0.0 0.0 5.0 9.5 R′₂O10.0 10.0 10.0 0.0 10.0 14.3 TiO₂ — — — — — — ZrO₂ — — — — — — SiO₂ +B₂O₃ + Al₂O₃ 75.0 70.0 70.0 72.0 82.0 85.7 B₂O₃/Al₂O₃ 2.0 1.0 1.0 0.81.7 2.5 RO + R′₂O 25.0 30.0 30.0 28.0 18.0 14.3 SiO₂ + B₂O₃ + Al₂O₃ +RO + R′₂O 100.0 100.0 100.0 100.0 100.0 100.0 SiO₂ + B₂O₃ + Al₂O₃ + R′₂O85.0 80.0 80.0 72.0 92.0 100.0 SiO₂ + B₂O₃ + Al₂O₃ + RO + 100.0 100.0100.0 100.0 100.0 100.0 R′₂O + TiO₂ + ZrO₂ Total 100.0 100.0 100.0 100.0100.0 100.0 Glass transition point Tg [° C.] 548 595 541 676 543 478 Sagtemperature Td [° C.] 625 676 596 733 614 551 Thermal expansioncoefficient α × 10⁻⁷[/° C.] 77 87 71 52 70 82 Density [g/cm³] 2.4242.518 2.540 2.598 2.336 2.292 Young's modulus E [GPa] 68.82 76.62 88.5386.1 61.54 53.42 Rigidity G [GPa] 27.79 31.13 35.74 — 24.84 21.63 Inwater Pressing load [gF] 1000 1000 1000 1000 1000 1000 Fragility indexvalue B [μm^(−1/2)] 6.9 9.3 9.7 9.6 5.7 6.6 Vickers Hardness Hv [GPa]5.1 5.5 5.7 5.9 4.5 4.2 Fracture toughness [MPa/m^(1/2)] 0.76 0.61 0.630.64 0.85 0.67 Probability 95 100 100 100 95 100 In dry Pressing load[gF] 5980 1000 1000 1000 6952 5980 atmosphere Fragility index value B[μm^(−1/2)] 4.5 5.9 6.1 5.4 3.4 3.4 N₂ Vickers hardness Hv [GPa] 4.9 5.56.1 5.7 4.4 4.2 Fracture roughness Kc [MPa/m^(1/2)] 1.17 0.96 1.03 1.121.40 1.31 Probability 80 100 100 85 80 40

TABLE 3 Examples 13 14 15 16 17 18 Composition SiO₂ 55.0 55.0 55.0 55.060.0 60.0 (mol %) B₂O₃ 10.0 15.0 20.0 25.0 10.0 10.0 Al₂O₃ 12.0 10.010.0 10.0 2.5 5.0 MgO 8.0 10.0 5.0 0.0 10.0 10.0 CaO 5.0 0.0 0.0 0.0 0.00.0 ZnO — — — — 0.0 0.0 RO 13.0 10.0 5.0 0.0 10.0 10.0 Li₂O 0.0 0.0 0.00.0 0.0 0.0 Na₂O 5.0 5.0 5.0 5.0 12.5 10.0 K₂O 5.0 5.0 5.0 5.0 5.0 5.0R′₂O 10.0 10.0 10.0 10.0 17.5 15.0 TiO₂ — — — — 0.0 0.0 ZrO₂ — — — — — —SiO₂ + B₂O₃ + Al₂O₃ 77.0 80.0 85.0 90.0 72.5 75.0 B₂O₃/Al₂O₃ 0.8 1.5 2.02.5 4.0 2.0 RO + R′₂O 23.0 20.0 15.0 10.0 27.5 25.0 SiO₂ + B₂O₃ +Al₂O₃ + RO + R′₂O 100.0 100.0 100.0 100.0 100.0 100.0 SiO₂ + B₂O₃ +Al₂O₃ + R′₂O 87.0 90.0 95.0 100.0 90.0 90.0 SiO₂ + B₂O₃ + Al₂O₃ + RO +100.0 100.0 100.0 100.0 100.0 100.0 R′₂O + TiO₂ + ZrO₂ Total 100.0 100.0100.0 100.0 100.0 100.0 Glass transition point Tg [° C.] 606 568 522 472536 541 Sag temperature Td [° C.] 678 659 612 558 595 606 Thermalexpansion coefficient α × 10⁻⁷[/° C.] 70 73 66 71 99 87 Density [g/cm³]2.439 2.365 2.307 2.238 2.470 2.442 Young's modulus E [GPa] 72.07 65.2158.72 49.65 73.94 72.54 Rigidity G [GPa] 29.43 26.60 23.83 20.02 30.3229.74 In water Pressing load [gF] 1000 1000 1000 1000 1000 1000Fragility index value B [μm^(−1/2)] 6.7 5.9 5.8 5.4 9.7 6.3 VickersHardness Hv [GPa] 5.0 4.8 4.4 3.9 5.1 5.1 Fracture toughness[MPa/m^(1/2)] 0.80 0.86 0.81 0.78 0.56 0.87 Probability 95 100 100 100100 100 In dry Pressing load [gF] 6952 5980 5980 5980 5980 5980atmosphere Fragility index value B [μm^(−1/2)] 4.4 3.6 3.6 3.1 4.9 4.4N₂ Vickers hardness Hv [GPa] 5.0 4.7 4.4 3.9 5.2 5.3 Fracture roughnessKc [MPa/m^(1/2)] 1.24 1.41 1.33 1.36 1.12 1.23 Probability 100 100 10060 100 80

TABLE 4 Examples 19 20 21 22 23 24 Composition SiO₂ 60.0 60.0 60.0 60.060.0 60.0 (mol %) B₂O₃ 10.0 10.0 10.0 10.0 10.0 10.0 Al₂O₃ 7.5 10.0 10.010.0 10.0 12.0 MgO 10.0 5.0 10.0 5.0 7.5 8.0 CaO 0.0 5.0 0.0 0.0 0.0 0.0ZnO 0.0 — — 5.0 0.0 — RO 10.0 10.0 10.0 10.0 7.5 8.0 Li₂O 0.0 0.0 0.00.0 0.0 0.0 Na₂O 7.5 5.0 5.0 5.0 5.0 5.0 K₂O 5.0 5.0 5.0 5.0 5.0 5.0R′₂O 12.5 10.0 10.0 10.0 10.0 10.0 TiO₂ 0.0 — — 0.0 2.5 — ZrO₂ 0.0 — —0.0 0.0 — SiO₂ + B₂O₃ + Al₂O₃ 77.5 80.0 80.0 80.0 80.0 82.0 B₂O₃/Al₂O₃1.3 1.0 1.0 1.0 1.0 0.8 RO + R′₂O 22.5 20.0 20.0 20.0 17.5 18.0 SiO₂ +B₂O₃ + Al₂O₃ + RO + R′₂O 100.0 100.0 100.0 100.0 97.5 100.0 SiO₂ +B₂O₃ + Al₂O₃ + R′₂O 90.0 90.0 90.0 90.0 90.0 92.0 SiO₂ + B₂O₃ + Al₂O₃ +RO + 100.0 100.0 100.0 100.0 100.0 100.0 R′₂O + TiO₂ + ZrO₂ Total 100.0100.0 100.0 100.0 100.0 100.0 Glass transition point Tg [° C.] 554 595583 567 566 598 Sag temperature Td [° C.] 619 676 686 672 672 686Thermal expansion coefficient α × 10⁻⁷ [/° C.] 85 73 77 72 77 66 Density[g/cm³] 2.409 2.419 2.382 2.448 2.386 2.347 Young's modulus E [GPa]69.70 70.63 67.26 66.36 66.05 62.00 Rigidity G [GPa] 28.54 29.05 27.6127.17 27.09 — In water Pressing load [gF] 1000 1000 1000 1000 1000 1000Fragility index value B [μm^(−1/2)] 6.5 6.5 6.1 5.9 5.4 6.0 VickersHardness Hv [GPa] 5.0 5.3 5.0 4.9 4.8 5.1 Fracture toughness[MPa/m^(1/2)] 0.82 0.83 0.85 0.87 0.95 0.83 Probability 100 100 100 95100 95 In dry Pressing load [gF] 5980 5980 5980 5980 5980 5980atmosphere Fragility index value B [μm^(−1/2)] 4.2 4.5 4.0 3.9 3.8 4.9N₂ Vickers hardness Hv [GPa] 5.0 5.2 5.0 4.9 4.8 4.9 Fracture roughnessKc[MPa/m^(1/2)] 1.27 1.19 1.29 1.32 1.34 1.01 Probability 60 100 100 10080 100

TABLE 5 Examples 25 26 27 28 29 30 Composition SiO₂ 60.0 60.0 65.0 65.065.0 65.0 (mol %) B₂O₃ 15.0 20.0 0.0 5.0 5.0 5.0 Al₂O₃ 10.0 10.0 7.0 2.55.0 5.0 MgO 5.0 0.0 1.0 10.0 10.0 5.0 CaO 0.0 0.0 1.0 0.0 0.0 5.0 ZnO —— 0.0 0.0 0.0 0.0 RO 5.0 0.0 2.0 10.0 10.0 10.0 Li₂O 0.0 0.0 10.0 0.00.0 0.0 Na₂O 5.0 5.0 10.5 12.5 10.0 10.0 K₂O 5.0 5.0 2.5 5.0 5.0 5.0R′₂O 10.0 10.0 23.0 17.5 15.0 15.0 TiO₂ — — 0.0 0.0 0.0 0.0 ZrO₂ — — 3.00.0 — 0.0 SiO₂ + B₂O₃ + Al₂O₃ 85.0 90.0 72.0 72.5 75.0 75.0 B₂O₃/Al₂O₃1.5 2.0 0.0 2.0 1.0 1.0 RO + R′₂O 15.0 10.0 25.0 27.5 25.0 25.0 SiO₂ +B₂O₃ + Al₂O₃ + RO + R′₂O 100.0 100.0 97.0 100.0 100.0 100.0 SiO₂ +B₂O₃ + Al₂O₃ + R′₂O 95.0 100.0 95.0 90.0 90.0 90.0 SiO₂ + B₂O₃ + Al₂O₃ +RO + 100.0 100.0 100.0 100.0 100.0 100.0 R′₂O + TiO₂ + ZrO₂ Total 100.0100.0 100.0 100.0 100.0 100.0 Glass transition point Tg [° C.] 540 488479 553 558 551 Sag temperature Td [° C.] 628 583 551 612.7 634 619Thermal expansion coefficient α × 10⁻⁷ [/° C.] 69 71 98 90 85 91 Density[g/cm³] 2.326 2.259 2.535 2.461 2.440 2.483 Young's modulus E [GPa]61.15 52.00 82.25 73.65 72.10 74.95 Rigidity G [GPa] 24.81 — 33.42 30.3529.73 30.81 In water Pressing load [gF] 1000 1000 1000 1000 1000 1000Fragility index value B [μm^(−1/2)] 6.3 6.2 9.5 7.7 7.6 9.9 VickersHardness Hv [GPa] 4.8 4.4 — 5.3 5.7 5.4 Fracture toughness [MPa/m^(1/2)]0.78 0.72 — 0.72 0.73 0.56 Probability 100 80 — 100 100 100 In dryPressing load [gF] 5980 5980 5980 5980 5980 5980 atmosphere Fragilityindex value B [μm^(−1/2)] 3.8 3.2 5.0 5.8 4.9 5.6 N₂ Vickers hardness Hv[GPa] 4.8 4.1 6.0 5.6 5.8 5.4 Fracture roughness Kc[MPa/m^(1/2)] 1.281.36 1.18 0.97 1.13 0.99 Probability 100 80 100 100 100 100

TABLE 6 Examples 31 32 33 34 35 36 Composition SiO₂ 65.0 65.0 65.0 65.065.0 65.0 (mol %) B₂O₃ 5.0 5.0 5.0 5.0 5.0 5.0 Al₂O₃ 5.0 5.0 5.0 5.0 5.05.0 MgO 5.0 5.0 5.0 5.0 5.0 5.0 CaO 3.0 3.0 5.0 5.0 5.0 5.0 ZnO — — — —— — RO 8.0 8.0 10.0 10.0 10.0 10.0 Li₂O 2.0 0.0 2.0 2.0 2.0 4.0 Na₂O10.0 12.0 11.0 9.0 7.0 9.0 K₂O 5.0 5.0 2.0 4.0 6.0 2.0 R′₂O 17.0 17.015.0 15.0 15.0 15.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 ZrO₂ — — — — — — SiO₂ +B₂O₃ + Al₂O₃ 75.0 75.0 75.0 75.0 75.0 75.0 B₂O₃/Al₂O₃ 1.0 1.0 1.0 1.01.0 1.0 RO + R′₂O 25.0 25.0 25.0 25.0 25.0 25.0 SiO₂ + B₂O₃ + Al₂O₃ +RO + R′₂O 100.0 100.0 100.0 100.0 100.0 100.0 SiO₂ + B₂O₃ + Al₂O₃ + R′₂O92.0 92.0 90.0 90.0 90.0 90.0 SiO₂ + B₂O₃ + Al₂O₃ + RO + 100.0 100.0100.0 100.0 100.0 100.0 R′₂O + TiO₂ + ZrO₂ Total 100.0 100.0 100.0 100.0100.0 100.0 Glass transition point Tg [° C.] 511 540 524 526 523 507 Sagtemperature Td [° C.] 581 608 599 593 595 572 Thermal expansioncoefficient α × 10⁻⁷ [/° C.] 92 95 83 85 87 106 Density [g/cm³] 2.4732.477 2.485 2.482 2.477 2.478 Young's modulus E [GPa] 76.71 74.00 78.9778.28 77.13 80.71 Rigidity G [GPa] 31.62 30.48 32.50 32.21 31.74 33.26In water Pressing load [gF] 1000 1000 1000 1000 1000 1000 Fragilityindex value B [μm^(−1/2)] 9.9 9.8 9.8 10.0 10.6 10.0 Vickers Hardness Hv[GPa] 5.6 5.3 5.7 5.8 5.8 5.8 Fracture toughness [MPa/m^(1/2)] 0.58 0.560.60 0.58 0.55 0.59 Probability 100 100 100 100 100 100 In dry Pressingload [gF] 5980 5980 5980 5980 5980 5980 atmosphere Fragility index valueB [μm^(−1/2)] 5.0 4.8 5.4 5.6 5.5 5.1 N₂ Vickers hardness Hv [GPa] 5.65.1 5.5 5.7 5.7 5.6 Fracture roughness Kc [MPa/m^(1/2)] 1.13 1.14 1.081.04 1.04 1.15 Probability 100 100 100 100 100 100

TABLE 7 Examples 37 38 39 40 41 42 Composition SiO₂ 65.0 65.0 65.0 65.065.0 65.0 (mol %) B₂O₃ 5.0 5.0 5.0 5.0 5.0 5.0 Al₂O₃ 5.0 5.0 5.0 5.0 5.05.0 MgO 5.0 5.0 5.0 5.0 5.0 5.0 CaO 5.0 5.0 5.0 5.0 5.0 5.0 ZnO — — — —— 0.0 RO 10.0 10.0 10.0 10.0 10.0 10.0 Li₂O 4.0 4.0 6.0 6.0 6.0 8.0 Na₂O7.0 5.0 7.0 5.0 3.0 5.0 K₂O 4.0 6.0 2.0 4.0 6.0 2.0 R′₂O 15.0 15.0 15.015.0 15.0 15.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 ZrO₂ — — — — — 0.0 SiO₂ +B₂O₃ + Al₂O₃ 75.0 75.0 75.0 75.0 75.0 75.0 B₂O₃/Al₂O₃ 1.0 1.0 1.0 1.01.0 1.0 RO + R′₂O 25.0 25.0 25.0 25.0 25.0 25.0 SiO₂ + B₂O₃ + Al₂O₃ +RO + R′₂O 100.0 100.0 100.0 100.0 100.0 100.0 SiO₂ + B₂O₃ + Al₂O₃ + R′₂O90.0 90.0 90.0 90.0 90.0 90.0 SiO₂ + B₂O₃ + Al₂O₃ +RO + 100.0 100.0100.0 100.0 100.0 100.0 R′₂O + TiO₂ + ZrO₂ Total 100.0 100.0 100.0 100.0100.0 100.0 Glass transition point Tg [° C.] 510 508 500 501 501 488 Sagtemperature Td [° C.] 579 579 566 568 571 552 Thermal expansioncoefficient α × 10⁻⁷ [/° C.] 85 115 78 81 80 78 Density [g/cm³] 2.4742.469 2.470 2.465 2.460 2.464 Young's modulus E [GPa] 79.81 78.63 82.3781.16 79.50 83.61 Rigidity G [GPa] 32.88 32.38 33.93 33.44 32.76 34.44In water Pressing load [gF] 1000 1000 1000 1000 1000 1000 Fragilityindex value B [μm^(−1/2)] 10.1 10.5 10.0 9.9 10.4 9.9 Vickers HardnessHv [GPa] 5.9 5.8 5.9 5.9 5.9 6.1 Fracture toughness [MPa/m^(1/2)] 0.580.56 0.60 0.60 0.57 0.61 Probability 100 100 100 100 100 100 In dryPressing load [gF] 5980 5980 5980 5980 5980 5980 atmosphere Fragilityindex value B [μm^(−1/2)] 5.6 5.1 4.9 5.0 5.2 4.7 N₂ Vickers hardness Hv[GPa] 5.7 5.5 5.6 5.6 5.6 5.5 Fracture roughness Kc [MPa/m^(1/2)] 1.041.13 1.13 1.17 1.11 1.25 Probability 100 100 100 100 100 100

TABLE 8 Examples 43 44 45 46 47 48 Composition SiO₂ 65.0 65.0 65.0 65.065.0 65.0 (mol %) B₂O₃ 5.0 5.0 5.0 5.0 5.0 5.0 Al₂O₃ 5.0 5.0 5.0 5.0 5.07.0 MgO 5.0 5.0 5.0 5.0 5.0 5.0 CaO 5.0 5.0 5.0 0.0 0.0 3.0 ZnO 0.0 0.00.0 0.0 0.0 — RO 10.0 10.0 10.0 5.0 5.0 8.0 Li₂O 8.0 8.0 0.0 8.0 2.0 0.0Na₂O 3.0 1.0 11.0 10.0 10.0 10.0 K₂O 4.0 6.0 4.0 2.0 5.0 5.0 R′₂O 15.015.0 15.0 20.0 17.0 15.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 ZrO₂ 0.0 — 0.0 0.03.0 — SiO₂ + B₂O₃ + Al₂O₃ 75.0 75.0 75.0 75.0 75.0 77.0 B₂O₃/Al₂O₃ 1.01.0 1.0 1.0 1.0 0.7 RO + R′₂O 25.0 25.0 25.0 25.0 22.0 23.0 SiO₂ +B₂O₃ + Al₂O₃ + RO + R′₂O 100.0 100.0 100.0 100.0 97.0 100.0 SiO₂ +B₂O₃ + Al₂O₃ + R′₂O 90.0 90.0 90.0 95.0 92.0 92.0 SiO₂ + B₂O₃ + Al₂O₃ +RO + 100.0 100.0 100.0 100.0 100.0 100.0 R′₂O + TiO₂ + ZrO₂ Total 100.0100.0 100.0 100.0 100.0 100.0 Glass transition point Tg [° C.] 494 504552 474 533 536 Sag temperature Td [° C.] 561 577 622 536 615 596Thermal expansion coefficient α × 10⁻⁷ [/° C.] 81 73 89 91 87 122Density [g/cm³] 2.458 2.446 2.485 2.449 2.525 2.462 Young's modulus E[GPa] 82.10 79.04 75.27 79.92 77.29 71.94 Rigidity G [GPa] 33.82 32.6430.96 32.45 31.61 29.58 In water Pressing load [gF] 1000 1000 1000 10001000 1000 Fragility index value B [μm^(−1/2)] 9.8 9.9 9.9 9.5 9.0 8.5Vickers Hardness Hv [GPa] 5.9 5.7 5.8 — — 5.1 Fracture toughness[MPa/m^(1/2)] 0.61 0.59 0.58 — — 0.63 Probability 100 100 100 — — 100 Indry Pressing load [gF] 5980 5980 5980 5980 5980 5980 atmosphereFragility index value B [μm^(−1/2)] 4.7 4.8 5.7 4.6 4.3 5.1 N₂ Vickershardness Hv [GPa] 5.6 5.4 5.5 5.6 5.5 5.2 Fracture roughness Kc[MPa/m^(1/2)] 1.25 1.19 0.99 1.27 1.30 1.06 Probability 100 100 100 100100 100

TABLE 9 Examples 49 50 51 52 53 54 55 Composition SiO₂ 65.0 65.0 65.065.0 65.4 65.4 65.4 (mol %) B₂O₃ 5.0 5.0 7.5 10.0 0.0 0.0 0.0 Al₂O₃ 7.510.0 2.5 0.0 8.6 8.6 8.6 MgO 10.0 10.0 5.0 5.0 0.0 0.0 0.0 CaO 0.0 0.05.0 5.0 0.0 0.0 0.0 ZnO 0.0 0.0 — — 0.0 0.0 0.0 RO 10.0 10.0 10.0 10.00.0 0.0 0.0 Li₂O 0.0 0.0 0.0 0.0 12.5 7.5 0.0 Na₂O 7.5 5.0 10.0 10.010.5 10.5 13.0 K₂O 5.0 5.0 5.0 5.0 0.0 5.0 10.0 R′₂O 12.5 10.0 15.0 15.023.0 23.0 23.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ZrO₂ — — — — 3.0 3.0 3.0SiO₂ + B₂O₃ + Al₂O₃ 77.5 80.0 75.0 75.0 74.0 74.0 74.0 B₂O₃/Al₂O₃ 0.70.5 3.0 — 0.0 0.0 0.0 RO + R′₂O 22.5 20.0 25.0 25.0 23.0 23.0 23.0SiO₂ + B₂O₃ + Al₂O₃ + RO + R′₂O 100.0 100.0 100.0 100.0 97.0 97.0 97.0SiO₂ + B₂O₃ + Al₂O₃ + R′₂O 90.0 90.0 90.0 90.0 97.0 97.0 97.0 SiO₂ +B₂O₃ + Al₂O₃ + RO + 100.0 100.0 100.0 100.0 100.0 100.0 100.0 R′₂O +TiO₂ + ZrO₂ Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 Glasstransition point Tg [° C.] 577 631 557 559 498 491 534 Sag temperatureTd [° C.] 658 752 622 623 564 568 629 Thermal expansion coefficient α ×10⁻⁷ [/° C.] 76 70 87 89 89 116 113 Density [g/cm³] 2.419 2.399 2.4862.493 2.511 2.523 2.537 Young's modulus E [GPa] 71.46 70.77 76.10 78.4184.13 79.70 71.31 Rigidity G [GPa] 29.51 29.49 31.35 32.37 34.71 32.8229.30 In water Pressing load [gF] 1000 1000 1000 1000 1000 1000 1000Fragility index value B [μm^(−1/2)] 8.4 7.1 10.4 13.2 7.8 8.4 7.9Vickers Hardness Hv [GPa] 5.6 5.1 5.4 5.9 5.6 5.7 5.4 Fracture toughness[MPa/m^(1/2)] 0.96 0.76 0.54 0.44 0.77 0.70 0.69 Probability 100 100 100100 100 100 100 In dry Pressing load [gF] 5980 5980 5980 5980 5980 59805980 atmosphere Fragility index value B [μm^(−1/2)] 5.2 5.3 4.9 5.5 4.35.0 5.2 N₂ Vickers hardness Hv [GPa] 5.6 5.2 5.2 5.5 5.4 5.5 4.9Fracture roughness Kc 1.05 1.01 1.13 1.05 1.37 1.16 1.02 [MPa/m^(1/2)]Probability 100 100 100 100 100 100 100

TABLE 10 Examples 56 57 58 59 60 61 Composition SiO₂ 48.0 53.2 57.0 59.059.0 59.0 (mol %) B₂O₃ 16.0 14.4 9.5 9.0 9.5 10.5 Al₂O₃ 16.0 14.4 9.59.0 9.5 10.5 MgO 0.0 0.0 2.0 3.0 2.0 0.0 CaO 0.0 0.0 0.0 0.0 0.0 0.0 ZnO0.0 0.0 0.0 0.0 0.0 0.0 RO 0.0 0.0 2.0 3.0 2.0 0.0 Li₂O 10.0 10.0 9.010.0 9.0 10.0 Na₂O 8.0 6.0 9.0 10.0 9.0 5.0 K₂O 2.0 2.0 2.0 0.0 2.0 5.0R′₂O 20.0 18.0 20.0 20.0 20.0 20.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 ZrO₂ 0.00.0 2.0 0.0 0.0 0.0 SiO₂ + B₂O₃ + Al₂O₃ 80.0 82.0 76.0 77.0 78.0 80.0B₂O₃/Al₂O₃ 1.0 1.0 1.0 1.0 1.0 1.0 RO + R′₂O 20.0 18.0 22.0 23.0 22.020.0 SiO₂ + B₂O₃ + Al₂O₃ + RO + R′₂O 100.0 100.0 98.0 100.0 100.0 100.0SiO₂ + B₂O₃ + Al₂O₃ + R′₂O 100.0 100.0 96.0 97.0 98.0 100.0 SiO₂ +B₂O₃ + Al₂O₃ + RO + 100.0 100.0 100.0 100.0 100.0 100.0 R′₂O + TiO₂ +ZrO₂ Total 100.0 100.0 100.0 100.0 100.0 100.0 Glass transition point Tg[° C.] 475 483 484 483 476 473 Sag temperature Td [° C.] 525 539 543 534530 529 Thermal expansion coefficient α × 10⁻⁷ [/° C.] 90 77 89 84 87 84Density [g/cm³] 2.382 2.359 2.492 2.456 2.444 2.415 Young's modulus E[GPa] 72.62 71.84 81.35 83.13 80.45 77.88 Rigidity G [GPa] 29.16 28.9632.96 33.70 32.73 31.71 In dry Pressing load [gF] 6952 6952 6952 69526952 6952 atmosphere Fragility index value B [μm^(−1/2)] 3.5 3.1 3.9 3.83.7 3.8 N₂ Vickers hardness Hv [GPa] 5.2 5.0 5.8 5.8 5.7 5.5 Fractureroughness Kc [MPa/m^(1/2)] 1.54 1.71 1.53 1.57 1.56 1.51 Probability 2060 100 100 100 80

TABLE 11 Examples 62 63 64 65 66 67 Composition SiO₂ 59.0 60.0 60.0 60.060.0 61.0 (mol %) B₂O₃ 10.5 0.0 5.0 7.5 9.0 5.0 Al₂O₃ 10.5 15.0 15.0 7.59.0 12.0 MgO 0.0 5.0 0.0 5.0 0.0 5.0 CaO 0.0 0.0 0.0 0.0 0.0 0.0 ZnO 0.00.0 0.0 0.0 0.0 0.0 RO 0.0 5.0 0.0 5.0 0.0 5.0 Li₂O 10.0 9.0 9.0 10.09.0 10.0 Na₂O 10.0 9.0 9.0 5.0 9.0 5.0 K₂O 0.0 2.0 2.0 5.0 2.0 2.0 R′₂O20.0 20.0 20.0 20.0 20.0 17.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 ZrO₂ 0.0 0.00.0 0.0 2.0 0.0 SiO₂ + B₂O₃ + Al₂O₃ 80.0 75.0 80.0 75.0 78.0 78.0B₂O₃/Al₂O₃ 1.0 0.0 0.3 1.0 1.0 0.4 RO + R′₂O 20.0 25.0 20.0 25.0 20.022.0 SiO₂ + B₂O₃ + Al₂O₃ + RO + R′₂O 100.0 100.0 100.0 100.0 98.0 100.0SiO₂ + B₂O₃ + Al₂O₃ + R′₂O 100.0 95.0 100.0 95.0 98.0 95.0 SiO₂ + B₂O₃ +Al₂O₃ + RO + 100.0 100.0 100.0 100.0 100.0 100.0 R′₂O + TiO₂ + ZrO₂Total 100.0 100.0 100.0 100.0 100.0 100.0 Glass transition point Tg [°C.] 485 530 491 464 487 495 Sag temperature Td [° C.] 536 610 560 535545 564 Thermal expansion coefficient α × 10⁻⁷ [/° C.] 82 93 88 91 87 78Density [g/cm³] 2.420 2.464 2.428 2.428 2.48 2.422 Young's modulus E[GPa] 80.13 82.79 78.75 79.23 81.53 80.56 Rigidity G [GPa] 32.65 33.7132.05 32.38 33.22 32.77 In dry Pressing load [gF] 6952 6952 6952 69526952 6952 atmosphere Fragility index value B [μm^(−1/2)] 3.5 4.2 4.1 4.23.9 3.9 N₂ Vickers hardness Hv [GPa] 5.5 5.7 5.4 5.8 5.8 5.5 Fractureroughness Kc [MPa/m^(1/2)] 1.65 — — 1.40 1.50 — Probability 75 80 100 6080 100

TABLE 12 Examples 68 69 70 71 72 73 Composition SiO₂ 61.0 62.0 63.0 63.063.0 65.0 (mol %) B₂O₃ 9.5 7.0 7.5 8.5 8.5 2.5 Al₂O₃ 9.5 7.0 7.5 8.5 8.510.5 MgO 0.0 2.0 2.0 0.0 0.0 5.0 CaO 0.0 — 0.0 0.0 0.0 0.0 ZnO 0.0 — 0.00.0 0.0 0.0 RO 0.0 2.0 2.0 0.0 0.0 5.0 Li₂O 10.0 9.0 9.0 10.0 9.0 10.0Na₂O 10.0 9.0 9.0 10.0 9.0 5.0 K₂O 0.0 2.0 2.0 0.0 2.0 2.0 R′₂O 20.020.0 20.0 20.0 20.0 17.0 TiO₂ 0.0 — 0.0 0.0 0.0 0.0 ZrO₂ 0.0 2.0 0.0 0.00.0 0.0 SiO₂ + B₂O₃ + Al₂O₃ 80.0 76.0 78.0 80.0 80.0 78.0 B₂O₃/Al₂O₃ 1.01.0 1.0 1.0 1.0 0.2 RO + R′₂O 20.0 22.0 22.0 20.0 20.0 22.0 SiO₂ +B₂O₃ + Al₂O₃ + RO + R′₂O 100.0 98.0 100.0 100.0 100.0 100.0 SiO₂ +B₂O₃ + Al₂O₃ + R′₂O 100.0 96.0 98.0 100.0 100.0 95.0 SiO₂ + B₂O₃ +Al₂O₃ + RO + 100.0 100.0 100.0 100.0 100.0 100.0 R′₂O + TiO₂ + ZrO₂Total 100.0 100.0 100.0 100.0 100.0 100.0 Glass transition point Tg [°C.] 483 480 471 480 479 493 Sag temperature Td [° C.] 535 540 528 533537 568 Thermal expansion coefficient α × 10⁻⁷ [/° C.] 85 86 89 84 89 80Density [g/cm³] 2.427 2.450 2.448 2.431 2.431 2.421 Young's modulus E[GPa] 81.00 83.00 80.96 81.57 79.91 80.67 Rigidity G [GPa] 33.06 — 33.0033.37 32.71 33.09 In dry Pressing load [gF] 6952 6952 6952 6952 69526952 atmosphere Fragility index value B [μm^(−1/2)] 3.7 4.0 4.1 3.9 3.84.1 N₂ Vickers hardness Hv [GPa] 5.6 — 5.7 5.5 5.5 5.7 Fractureroughness Kc [MPa/m^(1/2)] 1.58 — 1.44 1.51 1.52 — Probability 40 — 100100 100 100

TABLE 13 Examples 74 75 76 77 Composition SiO₂ 65.0 65.0 65.0 65.0 (mol%) B₂O₃ 5.0 7.5 7.5 7.5 Al₂O₃ 5.0 7.5 7.5 7.5 MgO 0.0 0.0 0.0 0.0 CaO0.0 0.0 0.0 0.0 ZnO 0.0 0.0 0.0 0.0 RO 0.0 0.0 0.0 0.0 Li₂O 10.0 5.010.0 10.0 Na₂O 10.0 10.0 5.0 7.5 K₂O 5.0 5.0 5.0 2.5 R′₂O 25.0 20.0 20.020.0 TiO₂ 0.0 0.0 0.0 0.0 ZrO₂ 0.0 0.0 0.0 0.0 SiO₂ + B₂O₃ + Al₂O₃ 75.080.0 80.0 80.0 B₂O₃/Al₂O₃ 1.0 1.0 1.0 1.0 RO + R′₂O 25.0 20.0 20.0 20.0SiO₂ + B₂O₃ + Al₂O₃ + RO + R′₂O 100.0 100.0 100.0 100.0 SiO₂ + B₂O₃ +Al₂O₃ + R′₂O 100.0 100.0 100.0 100.0 SiO₂ + B₂O₃ + Al₂O₃ + RO + 100.0100.0 100.0 100.0 R′₂O + TiO₂ + ZrO₂ Total 100.0 100.0 100.0 100.0 Glasstransition point Tg [° C.] 451 482 475 472 Sag temperature Td [° C.] 510541 538 529 Thermal expansion coefficient α × 10⁻⁷ [/° C.] 107 95 85 84Density [g/cm³] 2.453 2.452 2.420 2.429 Young's modulus E [GPa] 77.9877.81 78.83 80.62 Rigidity G [GPa] 31.79 31.87 32.27 33.01 In dryPressing load [gF] 6952 6952 6952 6952 atmosphere Fragility index valueB [μm^(−1/2)] 4.0 4.3 4.1 3.9 N₂ Vickers hardness Hv [GPa] 5.4 5.4 5.75.6 Fracture roughness Kc [MPa/m^(1/2)] 1.42 1.31 1.42 1.51 Probability100 100 100 80

TABLE 14 Examples 78 79 80 81 Composition SiO₂ 65.0 65.0 65.0 65.0 (mol%) B₂O₃ 7.5 7.5 7.5 10.0 Al₂O₃ 7.5 7.5 7.5 0.0 MgO 0.0 0.0 0.0 5.0 CaO0.0 0.0 0.0 0.0 ZnO 0.0 0.0 0.0 0.0 RO 0.0 0.0 0.0 5.0 Li₂O 10.0 10.09.0 5.0 Na₂O 10.0 10.0 9.0 10.0 K₂O 0.0 0.0 2.0 5.0 R′₂O 20.0 20.0 20.020.0 TiO₂ 0.0 0.0 0.0 0.0 ZrO₂ 0.0 0.0 0.0 0.0 SiO₂ + B₂O₃ + Al₂O₃ 80.080.0 80.0 75.0 B₂O₃/Al₂O₃ 1.0 1.0 1.0 — RO + R′₂O 20.0 20.0 20.0 25.0SiO₂ + B₂O₃ + Al₂O₃ + RO + R′₂O 100.0 100.0 100.0 100.0 SiO₂ + B₂O₃ +Al₂O₃ + R′₂O 100.0 100.0 100.0 95.0 SiO₂ + B₂O₃ + Al₂O₃ + RO + 100.0100.0 100.0 100.0 R′₂O + TiO₂ + ZrO₂ Total 100.0 100.0 100.0 100.0 Glasstransition point Tg [° C.] 483 478 477 484 Sag temperature Td [° C.] 544533 533 541 Thermal expansion coefficient α× 10⁻⁷ [/° C.] 83 83 85 98Density [g/cm³] 2.440 2.431 2.434 2.477 Young's modulus E [GPa] 82.0782.00 80.81 80.64 Rigidity G [GPa] 33.58 33.65 33.15 33.02 in dryPressing load [gF] 1000 1000 1000 1000 atmosphere Fragility index valueB [μm^(−1/2)] 3.9 3.8 3.9 4.2 N₂ Vickers hardness Hv [GPa] 5.6 5.6 5.75.7 Fracture roughness Kc [MPa/m^(1/2)] 1.52 1.53 1.48 1.39 Probability80 100 100 100

Comparative Examples 1-3

Table 15 shows physical properties of glasses described inJP-A-10-158028. TABLE 15 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 CompositionSiO₂ 69.28 69.94 68.94 (mol %) B₂O₃ — — — Al₂O₃ 2.5 2.51 4.24 MgO 6.976.96 5.77 CaO 7.97 8 8.28 SrO — 0.24 1.81 Li₂O — — — Na₂O 1.96 4.93 1.46K₂O 9.56 6.75 9.25 TiO₂ — — — ZrO₂ 1.76 0.66 0.26 Total 100.0 100.0100.0 Glass transition point Tg [° C.] 657 623 658 Sag temperature Td [°C.] 780 710 732 Thermal expansion 86.8 85.8 82.8 coefficient α × 10⁻⁷[/° C.] Density [g/cm³] 2.52 2.49 2.51 Young's modulus E [GPa] 80.5 80.381.5 Pressing load [gF] 1000 1000 1000 Fragility index value B[μm^(−1/2)] 7.2 7.2 7.1 Vickers hardness Hv [GPa] 6.1 6.1 6.2 Fractureroughness Kc [MPa/m^(1/2)] 0.90 0.91 1.05

Example 82

Using the glasses obtained in Examples 1-81, glass substrates for aninformation recording medium and magnetic disks were prepared byconsecutively carrying out (1) a preliminary lapping step, (2) a formprocessing step, (3) a precision lapping step, (4) a step of processingan end surface to form a mirror face, (5) a first polishing step, (6) asecond polishing step, (7) an inspection step and (8) a magnetic diskproduction step.

Pure water was used as a water in a polishing liquid for use withpolishing machines from the (4) step of processing an end-surface toform a mirror face to the (6) second polishing step.

(1) Preliminary Lapping Step

First, a disk-shaped glass substrate having a diameter of 96 mm and athickness of 1.5 mm was obtained from a molten glass by adirect-pressing method using an upper mold member, a lower mold memberand a sleeve. In this case, besides the direct-pressing, a down drawingmethod or a floating method may be employed to form a sheet glass, andthe sheet glass may be cut with a cutting grinder to obtain adisk-shaped glass substrate.

Then, the glass substrate was subjected to a lapping step for improvingthe glass substrate in dimensional accuracy and form accuracy. Thelapping step was carried out with a double-side lapping apparatus usingabrasive grains having a particle size of #400. Specifically, aluminaabrasive grains having a particle size of #400 were used, and a load ofapproximately 980 N was employed. And, both the surfaces of the glasssubstrate encased in a carrier were lapped to attain a surface accuracyof 0 to 1 μm and a surface roughness (R_(max)) of approximately 6 μm byrotating a sun gear and an internal gear.

(2) Form Processing Step

Then, a hole was made in the central potion of the glass substrate witha cylindrical grinder, and the circumferential (outer) end surface ofthe glass substrate was cut with the cylindrical grinder until the glasssubstrate had a diameter of 95 mm. Then, the outer end surface and theinner end surface of the glass substrate were chamfered. In this case,the end surfaces of the glass substrate had a surface roughness,R_(max), of approximately 4 μm.

(3) Precision Lapping Step

Then, the abrasive grains were changed to abrasive grains having aparticle size of #1,000, and the glass substrate surfaces were lapped,to attain a surface roughness, R_(max), of approximately 2 μm and asurface roughness, Ra, of approximately 0.2 μm. Then, the glasssubstrate that was finished with the above lapping step wasconsecutively immersed in a neutral detergent and water (withapplication of ultrasonic waves) to carry out ultrasonic cleaning.

(4) Step of Processing End Surface to Form Mirror Surface

Then, while the glass substrate was rotated, the (inner and outer) endsurfaces of the glass substrate were polished by brush-polishing untilthe end surfaces had a surface roughness, R_(max), of 1 μm and a surfaceroughness, Ra, of approximately 0.3 μm. Then, the glass substrate thatwas finished with the above mirror-surface processing of the endsurfaces was cleaned with water.

(5) First Polishing Step

Then, the first polishing step was carried out with a double-sidepolishing apparatus for removing scratches and strains that-were causedto remain by the above lapping step. In the double-side polishingapparatus, the glass substrate held with a carrier was intimately placedbetween polishing pads attached to upper and lower bases, and thecarrier was engaged with a sun gear and an internal gear to sandwich andpress the above glass substrate with the upper and lower bases.Thereafter, a polishing liquid was supplied into between the polishingpads and those surfaces of the glass substrate which were to bepolished, and the glass substrate was rotated, whereby the both thesurfaces of the glass substrate were concurrently polished while theglass substrate rotated while turning on its axis on the bases. As adouble-side polishing apparatus in Examples hereinafter, the sameapparatus was used.

Specifically, a hard polisher (hard polyurethane foam) was used as apolisher, and the polishing step was carried out. Polishing conditionswere as follows. Cerium oxide (average particle diameter 1.3 μm)+purewater were used as a polishing liquid. A load was set at 9.8 mN/mm², anda polishing time period was set for 15 minutes. The glass substrate thatwas finished with the above first polishing step was consecutivelyimmersed in washing vessels of neutral detergent, pure water, purewater, isopropyl alcohol (IPA) and IPA (vapor drying) to carry outultrasonic cleaning, and dried.

(6) Second Polishing Step

Then, the same double-side polishing apparatus as that used in the firstpolishing step was used, the polisher was replaced with a soft polisher(suede pads), and the second polishing step was carried out. The secondpolishing step was carried out, for example, for decreasing the surfaceroughness, Ra, to approximately 1.0 to 0.3 μm while maintaining the flatsurface obtained by the above first polishing step. Polishing conditionswere as follows. Cerium oxide (average particle diameter 0.8 μm)+purewater were used as a polishing liquid. A load was set at 9.8 mN/mm², anda polishing time period was set for 15 minutes. The glass substrate thatwas finished with the above second polishing step was consecutivelyimmersed in washing vessels of neutral detergent, pure water, purewater, IPA and IPA (vapor drying) to carry out ultrasonic cleaning, anddried.

The glass substrates that were processed as described above in thisExample were not chemically strengthened but did not undergo anyfracture during the above polishing steps and during handlings thereofbetween one step and another step.

(7) Inspection Step

The surfaces of the glass substrates that were finished with the abovedrying step were visually inspected and closely inspected by utilizinglight reflection, scattering and transmission. As a result, defects suchas scratches were found in the surfaces of the glass substrates.

The main surfaces of the glass substrates that were finished with theabove step were measured for surface roughness with an atomic forcemicroscope, to show that glass substrates for magnetic disks, whichglass substrates had an ultra-smooth surface having an R_(max) of 2.13nm and an Ra of 0.20 nm, were obtained.

(8) Magnetic Disk Production Step

An NiAl seed layer, a CrV undercoat layer, CoPtCrB magnetic layer and acarbon halide protective layer were consecutively formed on both themain surfaces of each of the glass substrates (for magnetic: disks)obtained through the above steps. Further, a perfluoropolyetherlubricant layer was formed by a dipping method, to give magnetic disks.The thus-obtained magnetic disks were tested for a touch down height, toshow an excellent value of 5 nm. Further, the magnetic disks weresubjected to a load-and-load test (100,000 times) to show that no headclashed.

Example 83

Magnetic disks were prepared in the same manner as in Example 82 exceptthat the following chemically strengthening step was carried out betweenthe (6) second polishing step and the (7) inspection step.

In the chemically strengthening step, a chemically strengtheningsolution containing a mixture of potassium nitrate and sodium nitratewas provided, the chemically strengthening solution was heated to 380°C., and the above cleaned and dried glass substrates were immersedtherein for approximately 4 hours for chemical strengthening. Thechemically strengthened glass substrates were consecutively immersed inwashing vessels of neutral detergent, pure water, pure water, IPA andIPA (vapor drying) to carry out ultrasonic cleaning, and dried.

A 0.4 mm thick thin piece was cut from each of the obtained glasssubstrates and measured through a polarizing microscope, to show thatchemically strengthened layers were formed. The glass substrates in thisExample did not undergo any fracture during the above polishing stepsand during handlings thereof between one step and another step.

The thus-obtained magnetic disks were tested for a touch down height, toshow an excellent value of 5 nm. Further, the magnetic disks weresubjected to a load-and-load test (100,000 times) to show that no headclashed.

The glass substrate for an information recording medium, provided by thepresent invention, is excellent in scratch resistance and light inweight, and has high resistance to progress of fractures or has highfracture toughness. As compared with any conventional glass substratefor an information recording medium, the glass substrate of the presentinvention can serve to greatly decrease a breakage during the productionof magnetic disks and during the use of an information recording medium.Further, since a glass for a glass substrate can be produced at a costequivalent to the cost required for commercially available glasssubstrates or at a low cost, the glass is promising as a less-expensivenext-generation glass substrate for a magnetic information recordingmedium.

1. A glass substrate for an information recording medium, having afragility index value, measured in water, of 12 μm^(−1/2) or less.
 2. Aglass substrate for an information recording medium, having a fragilityindex value, measured in an atmosphere having a dew point of −5° C. orlower, of 7 μm^(−1/2) or less.
 3. A glass substrate for an informationrecording medium, having a fragility index value, measured in water, of12 μm^(−1/2) or less and having a fragility index value, measured in anatmosphere having a dew point of −5° C. or lower, of 7 μm^(−1/2) orless.
 4. The glass substrate for an information recording medium asrecited in claim 1, comprising, by mol %, greater than 65%, as a total,of SiO₂ and at least one of B₂O₃ and Al₂O₃, 0 to 20% of RO in which R isat least one member selected from the group consisting of Ma, Ca, Zn, Srand Ba, 0 to 28% of R′₂O in which R′ is at least one member selectedfrom the group consisting of Li, Na and K, 0 to 10% of TiO₂ and 0 to 10%of ZrO₂, the total content of said components being at least 95 mol %.5. A glass substrate for an information recording medium, comprising, bymol %, 40 to 75% of SiO₂, 2 to 45% of B₂O₃ and/or Al₂O₃ and 0 to 40% ofR′₂O in which R′ is at least one member selected from the groupconsisting of Li, Na and K), wherein the total content of SiO₂, B₂O₃,Al₂O₃ and R′₂O is at least 90 mol %.
 6. The glass substrate for aninformation recording medium as recited in claim 5, having a fragilityindex value, measured in water, of 12 μm^(−1/2) or less.
 7. The glasssubstrate for an information recording medium as recited in claim 5,having a fragility index value, measured in an atmosphere having a dewpoint of −5° C. or lower, of 7 μm^(−1/2) or less.
 8. The glass substratefor an information recording medium as recited in claim 1, having aYoung's modulus of at least 70 GPa.
 9. The glass substrate for aninformation recording medium as recited in claim 1, having a modulus ofrigidity of at least 20 GPa.
 10. The glass substrate for an informationrecording medium as recited in claim 1, which is made of a glass havinga region wherein the glass has a viscosity of at least 1 Pa·s, in arange of temperatures equivalent to, and higher than, a liquidustemperature of the glass.
 11. The glass substrate for an informationrecording medium as recited in claim 1, which is made of a glass havinga thermal expansion coefficient of 60×10⁻⁷/° C. or greater at atemperature of from 100° C. to 300° C.
 12. The glass substrate for aninformation recording medium as recited in claim 1, which has nochemically strengthened layer.
 13. The glass substrate for aninformation recording medium as recited in claim 1, which has achemically strengthened layer.
 14. A magnetic information recordingmedium comprising a magnetic recording layer formed on the glasssubstrate for an information recording medium recited in claim 1.